Benzene is oxidized to maleic anhydride in the presence of catalyst. The active components of commonly used catalysts are vanadium oxides (see metal oxide catalysts). To prevent benzene from being completely oxidized, oxides of molybdenum, phosphorus, titanium, tungsten, silver and alkali metals are often added as additives, and inert substances with low specific surface area are used as catalyst supports, such as α－ Aluminum oxide, corundum, etc. The reaction is carried out under normal pressure at 350~400 ℃. The process consists of oxidation of benzene, separation and purification of maleic anhydride (see figure). Benzene vapor and air can form explosive mixture, so the concentration of benzene in the mixed gas entering the reactor should be beyond the explosion limit, generally 1%~1.4% (mole). Benzene oxidation is a strong exothermic reaction. Tubular fixed bed reactors are often used in industry. They have a large heat transfer surface. Outside the tubes, there is a cooling system. The reaction heat can be used to generate high-pressure steam. The gas leaving the reactor contains about 1% (mole) of maleic anhydride. About half of the maleic anhydride can be condensed into liquid by cooling, and the rest can be recovered by absorption. Absorbent water or inert organic solvent, most factories use water. The absorption solution obtained is an aqueous solution of maleic acid with a concentration of 35%~40% (mass). Azeotropic solvents (such as xylene and anisole) are required for dehydration to convert the acid back to anhydride. Dehydration can also be carried out in a membrane evaporator. Crude anhydride can be obtained by vacuum distillation. The yield of maleic anhydride in the whole process is 92%~96% (mass) calculated by benzene.

C4烃氧化法

C4 hydrocarbon oxidation method

正丁烷与丁烯均含有与顺酐相同的碳原子数，是生产顺酐的理想原料。由于正丁烷价格较其他原料低廉，用正丁烷更为有利。其流程与苯氧化法基本相同，催化剂为钒-磷-氧体系，添加剂有铁、铅、锌、铜、锑等元素的氧化物。可用固定床反应器或用流化床反应器，反应温度约400℃。正丁烷-空气混合物中正丁烷浓度为1.0%～1.6%(摩尔)。整个过程的顺酐收率按正丁烷计约为50%。由于C4烃氧化的选择性较低,因此设备投资较以苯为原料时为高，且后加工不能采用部分冷凝，而必须将反应气体中的顺酐全部用吸收法回收，从而使能耗加大。但由于正丁烷价格比苯便宜，而且苯毒性大。因此，以正丁烷为原料是有吸引力的，对本法所用催化剂的改进工作，各国都在大力进行中。

Both n-butane and butene contain the same number of carbon atoms as maleic anhydride. They are ideal raw materials for the production of maleic anhydride. Since the price of n-butane is lower than other raw materials, it is more advantageous to use n-butane. The process is basically the same as that of benzene oxidation. The catalyst is vanadium phosphorus oxygen system, and the additives are oxides of iron, lead, zinc, copper, antimony and other elements. Fixed bed reactor or fluidized bed reactor can be used, and the reaction temperature is about 400 ℃. The concentration of n-butane in the n-butane air mixture is 1.0% ～ 1.6% (mole). The yield of maleic anhydride in the whole process is about 50% based on n-butane. Due to the low selectivity of C4 hydrocarbon oxidation, the equipment investment is higher than that when benzene is used as raw material, and partial condensation cannot be used for post-processing. Instead, all maleic anhydride in the reaction gas must be recovered by absorption method, so as to increase energy consumption. However, n-butane is cheaper than benzene, and benzene is more toxic. Therefore, it is attractive to use n-butane as the raw material. The improvement of the catalyst used in this method is being vigorously carried out in various countries.

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